This invention relates to a method of inhibiting corrosion as well as inhibiting scale and deposit formation resulting from the saturation of mineral salts, and buildup of corrosion byproducts. The invention generally relates to the inhibition of corrosion of metals in contact with an electrolyte and the reduction of insulating films owing to scale formation/deposition. The invention most particularly relates to the inhibition of corrosion on metals which experience active-passive transition by application of a hydrogen peroxide donor. Illustrative of such metals are steel(s), aluminum, titanium or mixtures thereof; however the instant invention contemplates the treatment of any metal which is capable of experiencing active/passive transitions when exposed to passivating agents.
Corrosion of metals, particularly those metals found in cooling water circulating systems, and most particularly those in heat exchangers, are of critical concern.
In industrial cooling systems, water from rivers, lakes, ponds, wells, wastewater treatment plant effluent etc., is employed as the cooling media for heat exchangers. Such waters can contain a variety of either dissolved and/or suspended materials such as mineral salts, metals, organics, silt, mud etc.
The cooling water from a heat exchanger is typically passed through a cooling tower, spray pond or evaporative system prior to discharge or reuse. In such systems, cooling is achieved by evaporating a portion of the water passing through the system. Because of the evaporation that takes place during the cooling, both dissolved and suspended solids concentrate. The concentrating of various anionic ions such as chlorides and sulfates can increase the rate of corrosion of the metals making up the cooling system. This is especially true with the metals making up the heat exchangers that are experiencing higher temperatures.
Furthermore, contaminates such as hydrogen sulfide can also increase corrosion rates. Likewise, mineral salts, for example those of calcium and magnesium, can induce scaling of the heat exchanger. A scale common in cooling systems is calcium carbonate. Other scales or deposits such as calcium phosphate or iron can also inhibit heat transfer as well as induce under-deposit corrosion.
Deposit formation on heat exchangers seriously reduces heat transfer. Corrosion byproducts can form on the metal surface where a corrosion cell has formed. Deposits from metal oxides, silt, mud, microbiological activity, and process contamination can reduce the efficiency of heat transfer as well as increase corrosion.
Reducing the corrosion, scaling and deposition of heat exchangers and associated cooling system equipment is thus essential to the efficient and economical operation of a cooling water system. Excessive corrosion of the metallic surfaces can cause the premature failure of process equipment, necessitating down time for the replacement or repair of the equipment. Additionally, the buildup of corrosion products on the heat transfer surfaces impedes water flow and reduces heat transfer efficiency thereby limiting production or requiring down time for cleaning.
In order for corrosion to occur, a corrosion cell must form. The corrosion cell consist of two half cells, the cathode, and the anode.
The cathode is defined as the point where the reduction of a reducible substance takes place. In waters where the pH is greater than 4.2 (like that of a cooling water system), the primary reducible substance is oxygen. The steps involved with the cathode include: oxygen diffusion to the metal surface, adsorb by either physical or chemical adsorption, electron transfer, rearrangement with other adsorbed materials (i.e. water and electrons with subsequent formation of hydroxide ions), de-sorption of the newly formed byproduct (hydroxide), and diffusion into the bulk meter. With increased concentration of hydroxide ions, oxygen diffusion and adsorption rates are reduced.
The anode is defined as the point where dissolution of metal ions occurs. The dissolution of metal ions at the anode is a chemical process. The reaction forms ferrous hydroxide. Initially, the potential at the anode is low; however with time, the electrical potential at the anode increases. The increased potential is the result of the increased concentration of metal ions (result of dissolution) in the immediate vicinity of the anode. The increased concentration of metal ions induces the formation of a Metal Ion Concentration Cell, as well as the reduction of oxygen. The increasing concentration of cationic charged ions at the anode increases the electrical potential of the anode.
General corrosion is defined as a state where the potential of the cathode decreases with time while the potential at the anode increases. At some point, the potentials of the cathode and anode shift or find neighboring electrodes of stronger or weaker potential. This shifting or jumping is the result of the mechanisms already described. As the hydroxide concentration at the cathode increases, oxygen adsorption decreases, and the cathodes potential goes down. At the anode, where the concentration of cationic ions increases, the demand for electrons increases, so the potential goes up. This process of electrode reversal continues across the surface of the metal resulting in a uniform loss of metal.
Pitting corrosion refers to a condition where the potential surrounding the anode is high (cathodic) and electron flow is not distributed across many anodes, therefore the electron comes from a local anode. Pitting corrosion is of great concern because of the high loss of metal from a localized area. At a metal loss rate of several mils per year (MPY), general corrosion would take many decades of continued corrosion before resulting in failure of the part, e.g. of a heat exchanger. However, in pitting corrosion, the electron flow and subsequent metal loss is from a localized area. Pitting corrosion often results in equipment failure long before reaching the expected life of the equipment, e.g. the heat exchanger.
Pitting corrosion occurs when the cathodic surface has been depolarized. Chlorides for example, compete for the metal surface with the oxygen donor. When a chloride ion is adsorbed at the metal surface, it prevents the oxygen from reaching the surface. The potential at the site is reduced, and the area becomes anodic. This induces a high flux of electrons to flow from the localized site to the surrounding cathode.
Passivation can be defined as the loss of chemical reactivity exhibited by certain metals and alloys under specific environmental conditions. The onset of passivation is associated with the formation of an oxide layer that is resistant to further oxidation.
The mechanism of passivation, as it relates to ferrous metal surfaces, involves the dissolution of metal ions, followed by formation of a ferrous hydroxide layer, followed by conversion to an insoluble ferric oxide by reaction with oxygen. Analysis of passive films indicates a layered structure with an outer layer of gamma iron oxide and an inner layer of magnetite.
Passivation occurs when we have established sufficient oxidation potential. At low oxidation potential, insufficient concentrations of oxidizer exist to establish a homogenous oxide layer; distinct anodes and cathodes exist. When sufficient oxidizer is present, the electrical potential of the entire surface is increased. The concentration of oxidizer is sufficient to induce the xe2x80x9cflashxe2x80x9d formation of an oxide layer. In other words, the oxidizer concentration is sufficiently high to react with the ferrous hydroxide across the entire metal surface. With increased oxidation potential, the current density increases. The current density required to induce passivation is called the critical current. At yet higher oxidizer concentrations, aggressive attack of the oxide layer occurs, and the corrosion rates increase; this is termed the transpassive region.
Passivation is a process requiring oxygen. Therefore, inhibitors that induce passivation must increase the oxidizing potential or make better use of the oxidizer present.
Inhibitors such as chromate, nitrite and hydroxide raise the oxidizing potential of the solution by directly supplying oxygen to the anode to react with the metal ions thereby reducing the formation of a metal oxide. For this reason, these inhibitors can be used in solutions without oxygen present. These oxidizing inhibitors are involved with direct oxidation of the ferrous hydroxide to ferric oxides.
Inhibitors such as phosphate, silicate, borate and molybdate are non-oxidizing inhibitors which form complexes with iron (III) species so as to stabilize them from disproportionation (the transformation of a substance into two or more substances by oxidation or reduction) to the more soluble Fe (II) species until reaction with dissolved oxygen creates the more immune oxides. These inhibitors do require the presence of oxygen to promote passivation. Their performance can be enhanced with the addition of a peroxide compound where peroxide is used to enhance the formation of a passive oxide film.
Anodic inhibition is desired from an operational viewpoint since the oxide film is very thin, and the corrosion rates are lower than that achieved by cathodic inhibition. However, the potential drawback is that if the film breaks are not repaired, due to insufficient inhibitor concentrations for example, rapid pitting may occur. This is the result of the fact that the cathodic reaction can occur all over the passive surface, but the anodic reaction (metal dissolution) can only occur at broken film sites.
Chromate treatments have been long recognized as the standard to compare other treatment programs. Chromates, induce the formation of a uniform oxide film thereby inducing passivation of the metal surface.
While chromate programs could be run at higher pH, many were operated at lower pH (6.5-7.5). This significantly lowered the potential for scale formation.
Chromate technology has been eliminated from most applications involving cooling water treatment due to the environmental and human health hazards. Since the elimination of chromates, the most common treatment methodologies now incorporate technologies that increase scale and fouling potential.
Anodic phosphate treatments require levels of phosphate exceeding those allowed by the natural solubility of phosphate when in the presence of calcium. To stabilize phosphates and inhibit scale formation, polymers that stabilize the formation of calcium phosphate salts are applied. Also, in some areas, lower phosphate levels are required to meet environmental restrictions.
Low phosphate levels can be achieved by incorporating a cathodic inhibitor like zinc. Zinc is a cathodic inhibitor that precipitates at the cathode by forming zinc hydroxide. Concentrations of peroxide and peroxycarboxylic acids can be reduced with the addition of a cathodic inhibitor such as zinc.
Again, because of the limited solubility of zinc, polymers are commonly fed to help prevent fouling from precipitation. Also, in many areas, zinc has environmental implications.
Using existing treatment methods, operating cost increased, and in many operating conditions such as high heat flux and/or high chlorides, results are less than desirable.
Thus, what is needed in the industry is a treatment program that is effective at inhibiting corrosion under various heat flux and water chemistry conditions, as well as being environmentally innocuous.
U.S. Pat. No. 5,800,732 describes the use of hydrogen peroxide as a means of controlling microbial activity in an all-in-one treatment for cooling water.
U.S. Pat. No. 4,977,292 describes the development of an organic phosphorous compound using hydrogen peroxide as an oxidizer to initiate the oxidation reaction.
U.S. Pat. No. 5,382,367 describes the use of hydrogen peroxide in cooling water systems to control microbiological activity in said systems.
U.S. Pat. Nos. 5,980,758, 5,785,867, 5,658,467 and 5,494,588 describe the use of peracetic acid with nonoxidizing biocides to inhibit growth of microorganisms.
The prior art fails to describe the use of peroxide and/or peroxycarboxylic acids as standalone corrosion inhibitors, much less passivating agents, for cooling water treatment. Furthermore, no prior art reference describes the use of these compounds as on-line deposit control and deposit removal agents for said application.
The instant invention is directed toward a method which provides superior corrosion inhibition, particularly to those areas experiencing heat transfer; namely the heat exchangers where corrosion and scale potential is most prevalent. The instant process recognizes the fact that both hydrogen peroxide and peroxycarboxylic acids are strong oxidizing agents having important similarities to chromate (VI) in that they are anionic oxidizers.
Since they function as anionic oxidizers, these compounds have high solubility in water and readily diffuse to the metal surface. Being strong oxidizers as well as oxygen donors, they are able to directly supply the oxygen needed to induce passivation.
The instant process utilizes these compounds as corrosion inhibitors and passivators to achieve performance equal to or exceeding that obtained by chromate. Such performance has been demonstrated under high heat load conditions and/or in the presence of corrosion inducing ions such as chlorides. These compounds also provide an environmentally innocuous treatment that eliminates the need for other corrosion inhibitors such as zinc and phosphate based treatments, as well as their supporting polymer treatment. The instant invention teaches an innovative treatment technology having application wherever removal of scale or deposits in contact with an electrolyte is required. The invention also teaches effective removal of insulating films while inhibiting corrosion (especially under heat transfer) and can be effectively used to improve operational performance without suffering loss of system integrity or production time.
The chemistry selected for removal of the insulating layers is based on the composition of the deposits. In the test, ferrous oxide was the primary composition and therefore, peroxycitric acid solution or mineral acid was affective at removing the deposit. Other chelating, sequestering, and dispersing agents could also provide effective results when used in conjunction with peroxycarboxylic acid(s) solutions.
Accordingly, it is an objective of the instant invention to utilize hydrogen peroxide donors as corrosion inhibitors and passivators to achieve performance equal to or exceeding that obtained by chromate.
It is a further objective of the instant invention to provide an environmentally innocuous treatment that eliminates the need for other corrosion inhibitors such as zinc and phosphate based treatments, as well as their supporting polymer treatment.
It is yet another objective of the instant invention to teach a treatment technology having application wherever removal of scale or deposits in contact with an electrolyte is required.
It is a still further objective of the invention to teach effective removal of insulating films while inhibiting corrosion (especially under heat transfer) thereby improving operational performance without suffering loss of system integrity or production time.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.